One known type of a conventional liquid crystal display device is a liquid crystal display device including a liquid crystal layer formed by a ferroelectric liquid crystal (hereinafter referred to as FLC) between transparent substrates respectively having scanning electrodes and signal electrodes arranged in a checker board pattern. Since FLC has spontaneous polarization, the aligned state of molecules has bistability.
The liquid crystal display device using FLC can perform writing at higher speeds compared to a conventional simple matrix liquid crystal display device using nematic liquid crystals or other types of liquid crystals. Such an advantage is gained because the aligned state of FLC is switched by the mutual function of an applied electric field and spontaneous polarization, and FLC molecules have a so-called memory characteristic whereby an aligned state in which an electric field is being applied is retained even after the electric field disappears.
FLC can be classified into two types depending on whether its dielectric anisotropy is positive or negative. The significant difference between these two types of FLC is the relationship between a pulse width (.tau.) of a drive voltage that effects switching (changes the aligned state) and a pulse height (V), i.e., the .tau.-V characteristic. FIG. 7(a) is a graph showing the .tau.-V characteristic of FLC having positive dielectric anisotropy, and FIG. 7(b) is a graph showing the .tau.-V characteristic of FLC having negative dielectric anisotropy.
For example, in an ideal driving characteristic when a monopulse is applied as a drive pulse, if the coordinate indicating a combination of the pulse width and pulse height of the drive pulse applied to FLC belongs to a region above the characteristic curve (.tau.-V curve) shown in FIGS. 7(a) and 7(b), switching occurs. On the other hand, if the coordinate belongs to a region below the above-mentioned characteristic curve, switching does not occur. For instance, in FLC having positive dielectric anisotropy, when a drive pulse having a uniform pulse width .tau..sub.1 shown in FIG. 7(a) is applied, if the pulse height of the drive pulse is V.sub.2 (V.sub.2 =V.sub.1 -V.sub.d) shown in FIG. 7(a), switching does not occur. In this case, switching occurs when the pulse height of the drive pulse is V.sub.3 (V.sub.3 =V.sub.1 +V.sub.d).
As is clear from a comparison between FIGS. 7(a) and 7(b), in FLC having positive dielectric anisotropy, as the pulse height of the drive pulse increases, the pulse width necessary for switching decreases monotonously. In contrast, in FLC with negative dielectric anisotropy, a pulse height (V.sub.min) at which the pulse width necessary for switching becomes minimum is present, i.e., a so-called .tau.-V.sub.min characteristic is exhibited. Moreover, the slope of the .tau.-V curve is steeper on a high voltage side higher than V.sub.min than on a low voltage side lower than V.sub.min.
Therefore, in FLC with negative dielectric anisotropy, when a drive pulse having a uniform pulse width .tau..sub.2 shown in FIG. 7(b) is applied, if the pulse height of the drive pulse is V.sub.5 (V.sub.5 =V.sub.4 -V.sub.d) or V.sub.9 (V.sub.9 =V.sub.7 +V.sub.d) shown in FIG. 7(b), switching does not occur. In this case, switching occurs when the pulse height of the drive pulse is V.sub.6 (V.sub.6 =V.sub.4 +V.sub.d) or V.sub.8 (V.sub.8 =V.sub.7 -V.sub.d).
More specifically, FLC with negative dielectric anisotropy can be driven by two types of drive schemes: low-voltage drive scheme, and high-voltage drive scheme. In the low-voltage drive scheme, driving is performed by a drive pulse for causing a non-writing voltage (a pulse height that does not effect a switching of FLC) to be applied to a pixel in a selecting period to be lower than a writing voltage (a pulse height which effects a switching of FLC) like the V.sub.5 -V.sub.6 relationship. On the other hand, in the high-voltage drive scheme, the non-writing voltage becomes higher than the writing voltage like the V.sub.8 -V.sub.9 relationship.
As described above, the slope of the .tau.-V curve on the high voltage side higher than V.sub.min is steep. Hence, an advantage of the high-voltage drive scheme is a wide tolerance of slot time due to a big difference between a response speed to the non-writing voltage and a response speed to the writing voltage.
As the low-voltage drive scheme, various schemes are disclosed in "ADDRESSING SCHEMES FOR FERROELECTRIC LIQUID CRYSTAL MATRIX DISPLAYS", C. T. H. Yeoh et al., Ferroelectrics, Vol. 132, pages 293-307 (1992).
On the other hand, examples of the high-voltage drive scheme include the JOERS/Alvey drive scheme disclosed in THE "JOERS/ALVEY" FERROELECTRIC MULTIPLEXING SCHEME, P. W. H. Surguy et al., Ferroelectrics, Vol. 122, pages 63-79 (1991), and so-called Malvern drive scheme described in a laid-open PCT international application, No. WO 92/02925.
The Malvern drive scheme is usually called together with the number of slots of a strobe pulse. For instance, when the strobe pulse is a two-slot strobe, the scheme is called the Malvern-2 scheme. Similarly, when the strobe pulse is a three-slot strobe, it is called the Malvern-3 scheme.
The FLC drive schemes are also classified into two groups: the two-field drive scheme, and the blanking drive scheme. In the two-field drive scheme, one frame period is composed of two fields. In the blanking drive scheme, one frame period is composed of only one field. When FLC is fixed in one stable state for a long time by a DC voltage, it tends not to switch to the other stable state, causing a probability of deterioration of bistability. In order to avoid such an unfavorable condition, in the two-field drive scheme and the blanking drive scheme, the voltages to be applied to the pixels are averaged within one frame.
More specifically, in the two-field drive scheme, when a drive voltage for writing or retaining the alignment of FLC in one of the stable states is applied in the first field, a drive voltage for writing or retaining the alignment of FLC in the other bistable state is applied in the following second field.
On the other hand, in the blanking drive scheme, the alignment of FLC in the pixel region is changed to predetermined one of bistable states by giving the blanking pulse to the scanning electrode before the selecting period so as to blank the pixel. The blanking drive scheme has such an advantage over the two-field drive scheme that the scanning time is reduced to about a half because one frame is formed by one field.
One known conventional blanking drive scheme for FLC having the .tau.-V.sub.min characteristic is called the CY drive scheme. The CY drive scheme is disclosed in the above-mentioned publication "Writing SCHEMES FOR FERROELECTRIC LIQUID CRYSTAL MATRIX DISPLAYS", C. T. H. Yeoh et al., Ferroelectrics, vol. 132, pages 293-307 (1992), and Japanese Publication for Unexamined Patent Application (Tokukaihei) No. 5-249434 (1993). The CY drive scheme is discriminated from the present invention by its characteristic that the blanking pulse has a pulse width which is just twice that of the strobe pulse.
Next, the following description will discuss the application of typical conventional two-field drive schemes such as the JOERS/Alvey drive scheme to the blanking drive scheme. More specifically, a strobe pulse having the same shape as the strobe pulse used in the two-field drive schemes is used, and one frame period is formed by one field by applying a blanking pulse having opposite polarity and equal pulse area to the scanning electrode before the selecting period so as to shorten the scanning time. The data signal to be applied to the signal electrode satisfies the same conditions as those in the two-field drive schemes. A liquid crystal display device using FLC with negative dielectric anisotropy was driven by such blanking drive schemes.
However, as to be explained in detail below, it was found that, in these blanking drive schemes, the tolerance of slot time becomes narrower and accurate switching may not be effected depending on the waveform of the data signal. The slot time referred here is a time forming a single unit of the pulse width of the drive pulse.
First, referring to FIGS. 8 to 10, the following description will explain driving of a liquid crystal display device by a blanking drive scheme adapting the JOERS/Alvey drive scheme.
Shown at the top of FIG. 8 is the waveform of the scanning signal to be applied to the scanning electrode by this blanking drive scheme. As is clear from the waveform, in this blanking drive scheme, one frame period includes a selecting period composed of two slots (2.tau..sub.s) and a blanking period of two slots having the same length as the selecting period before the selecting period.
The pulse height of the first slot in the selecting period is 0 V, and a strobe pulse having a pulse height V.sub.s is applied in the second slot. In the blanking period, a blanking pulse having opposite polarity to the above-mentioned strobe pulse, a pulse width of two slots and pulse height V.sub.b is applied.
In this case, the coefficient, .alpha., of the pulse height defined as EQU .alpha.=.vertline.V.sub.b /V.sub.s .vertline.,
and the pulse height and the pulse width of the blanking pulse are determined so that the pulse area of the blanking pulse and the pulse area of the strobe pulse are equal to each other and .alpha. is 0.5. The pulse area is the product of the pulse width and the pulse height.
The data signal to be applied to the signal electrode is the same as that in the conventional JOERS/Alvey drive scheme, and represented by bipolar pulses having a cycle of two slots like an example shown in the middle in FIG. 8. When the data signal represents writing data, -V.sub.d is applied in the first slot and +V.sub.d is applied in the second slot. On the other hand, when the data signal represents non-writing data, +V.sub.d is applied in the first slot and -V.sub.d is applied in the second slot.
A potential difference between the scanning signal and the data signal is applied as a drive pulse to the pixel. The waveform of the drive pulse produced at the pixel by the scanning signal and the data signal is shown at the bottom in FIG. 8.
With the use of the drive pulse, a minimum pulse width .tau..sub.e that permits switching by the blanking voltage to be applied to the pixel in the blanking period, a minimum pulse width .tau..sub.w that permits switching by the writing voltage to be applied to the pixel in the selecting period, a maximum pulse width .tau..sub.n that does not permit switching by the non-writing voltage, were measured against the number, K, of slots between the blanking period and the selecting period. The results of the measurements are shown in FIG. 9. In FIG. 9, curves 51, 52, and 53 indicate .tau..sub.n, .tau..sub.e, and .tau..sub.w, respectively.
The minimum (.tau..sub.min) of the slot time (.tau..sub.s) that can effect switching with accuracy is given by a larger pulse width between .tau..sub.e and .tau..sub.w. On the other hand, the maximum (.tau..sub.max) of the slot time (.tau..sub.s) is given by .tau..sub.n. Accordingly, the tolerance of the slot time .tau..sub.s is a region indicated by hatching in FIG. 9.
Broken lines 54 and 55 in FIG. 9 represent the maximum and minimum of the slot time, respectively, based on the JOERS/Alvey drive scheme as the two-field drive scheme. In the two-field drive scheme, the minimum of the slot time is given by a minimum pulse width that permits switching by the writing voltage.
As is clear from FIG. 9, in the blanking drive scheme adapting the JOERS/Alvey drive scheme, the value of the minimum of the slot time increases compared to the original two-field drive scheme. Moreover, the value of the maximum of the slot time decreases compared to the original two-field drive scheme. Thus, the tolerance of the slot time becomes narrower considerably.
Here, the problems caused by the narrower tolerance of the slot time will be discussed. One of the characteristics of FLC is that the response speed to the drive voltage varies depending on a change in the ambient temperature. FIG. 10 is a graph showing how the minimum and maximum of the slot time vary depending on the ambient temperature by fixing the number, K, of slots between the blanking period and the selecting period to a uniform value. Curves 61 and 62 in FIG. 9 indicate changes in the maximum .tau..sub.max and the minimum .tau..sub.min, respectively.
When the slot time is fixed to a given value .tau..sub.s, the range of ambient temperature within which driving can be performed with .tau..sub.s is between T.sub.min and T.sub.max shown in FIG. 10. Namely, as the space between the curves 61 and 62 is reduced, i.e., as the tolerance of the slot time becomes narrower, the temperature range within which driving can be performed becomes narrower. When the ambient temperature is out of the above-mentioned range, switching of FLC molecules is imperfect, causing problems such as a lowered contrast ratio and improper display.
In addition, another blanking drive scheme was tested by applying to the scanning electrode a strobe pulse with a pulse width of not less than two slots in a trailing part of the selecting period and a period that follows the selecting period and applying to the scanning electrode a blanking pulse having a polarity opposite to and the same pulse area as the strobe pulse so as to erase the pixel like the above-mentioned Malvern drive scheme. In this scheme, even when the data signal applied to the signal electrode in the selecting period was the non-writing data, switching of FLC sometimes occurred in the vicinity of the selecting period. Accordingly, this scheme suffers from a drawback that there is a possibility of failing to obtain a desired display result.